Photolithography is used to produce electronic optical or mechanical microstructures, or microstructures combining electronic and/or optical and/or mechanical functions. It consists in irradiating, with photon radiation, through a mask that defines the desired pattern, a photosensitive resist or photoresist layer deposited on a planar substrate (for example a silicon wafer). The chemical development that follows the irradiation reveals the desired patterns in the resist. The resist pattern thus etched may serve both for several purposes, the most common being the etching of an underlying layer (whether insulating or conducting or semiconducting) so as to define a pattern identical to that of the resist in this layer.
It is sought to obtain extremely small and precise patterns and to align etched patterns very precisely in multiple superposed layers. Typically, the critical dimension of the desired patterns is nowadays a fraction of a micron, or even a tenth of a micron and below. The wavelength of the light used in the photolithography operation limits the resolution—the shorter the wavelength, the finer the patterns may be. Ultraviolet photolithography (and wavelengths down to 193 nanometers) allows finer features to be produced than with visible light.
It is endeavoured at the present time to go well below these wavelengths and to work in extreme ultraviolet (EUV) at wavelengths between 10 and 14 nanometers. The objective is to obtain a very high resolution, while still maintaining a low numerical aperture and a sufficient depth of field (a few hundred nanometers).
However, at these wavelengths the materials forming the substrate of the mask are not transparent and the photolithography operation must use masks operating in reflection and not in transmission: the extreme ultraviolet light is projected onto the mask at a low angle of incidence (about 5 to 6 degrees); the mask comprises absorbent zones and reflecting zones; in the reflecting zones, the mask reflects the light onto the resist to be exposed, impressing its image thereon. The path of the light between the mask and the resist to be exposed passes via other reflectors, the geometries of which are designed so as to project a reduced image of the mask and not a full-size image. The image reduction makes it possible to etch smaller patterns on the exposed resist than those etched on the mask.
The mask itself is fabricated using a photolithography process followed by etching with a resist mask or a hard mask (for example made of silica, silicon nitride or chromium), this time in transmission and with a longer wavelength, permitted by the fact that the features are larger.
Typically, a reflection mask of the binary mask type is made up of a planar substrate covered with a continuous reflecting structure, in practice a Bragg mirror i.e. a structure consisting of multiple dielectric layers of different refractive indices, the thicknesses of which are calculated according to the indices, the wavelength and the angle of incidence, so that the various partially reflecting interfaces reflect light waves in phase with one another. This mirror is covered with an absorbent layer etched in the desired masking pattern so that the mask comprises reflecting zones (mirrors not covered with absorbent) and absorbent zones (mirrors covered with absorbent). To give an example, for a wavelength of 13.5 nm and an angle of incidence of 6 degrees, a reflectivity of 74% is achieved with forty silicon layers of 41.5 ångström (1 ångström=0.1 nm) thickness alternating with forty layers of molybdenum of 28 ångström thickness. The absorbent zones may consist (among others) of chromium deposited on the mirror. For example, a 600 ångström chromium layer placed on the above mirror now reflects only 1% of the incident light.
Among the drawbacks of this mask structure is notably the fact that the absorbent zones are thick (several hundred nanometers), resulting in not insignificant shadowing of the transitions between absorbent and reflecting zones in the presence of oblique or even low-angle (6°) illumination, and hence a loss of resolution.
It has also been proposed to produce the absorbent zones by locally hollowing out the surface of the mirror and filling the opening with an absorbent layer. The shadowing effect is reduced, but the process is very complex. The article “Design and Method of Fabricating Phase Shift Masks for Extreme Ultraviolet Lithography by Partial Etching into the EUV Multilayer Mirror” by Sang-In Han et al., in Proceedings of the SPIE, vol. 5037 (2003), describes such a structure.
Structures have also been proposed for what are called “EUV-PSM masks” (PSM standing for “Phase Shift Mask”). Openings are cut out in the mirror so as to locally reduce its thickness by a value such that the light reflected in the zones of reduced thickness are in phase opposition with the light reflected in the zones of nonreduced thickness. This creates, at the boundary, destructive interference equivalent to absorbent zones. If the reflection coefficient R2 in the zones of reduced thickness remains close to the reflection coefficient R1 in the zones of nonreduced thickness, (for example R2 equal to 85% of R1 or more), the term “H-PSM” (Hard Phase Shift Mask) is used. If one of the reflection coefficients is considerably lower than the other, the term “A-PSM” (Attenuated Phase Shift Mask) is used.
There are also what are called “Alt-PSM” masks or alternating phase shift masks in which two close reflecting zones are separated by an absorbent zone, the two successive reflecting zones producing reflections in phase opposition, thereby ensuring that the reflected light intensity systematically passes through zero in an absorbent zone.
All these mask structures are complex and therefore expensive to produce accurately, or else they create a substantial relief on the surface (and therefore shadowing in the presence of oblique illumination), in particular because of the necessary thickness of absorbent in the dark zones.